XX. Measurement of Feed and Nutrient Utilization in Food-Producing Animals

To develop feeding standards for food-producing animals, a knowledge and understanding of nutrients in the feed and its utilization by the animal is needed. Several animal and feed factors can affect nutrient utilization. These include species (ruminant vs. monogastrics), age (young vs. old), physiological (young vs. pregnant or lactating) state of the animal, disease conditions, type of feed or processing (pellet vs. ground), and presence of antinutritional factors in feed.

Methods used for nutrient evaluation of feed are in general similar for all classes of livestock. In this chapter, some of the common methods used for assessing nutrient utilization are addressed.

Growth trials are often conducted to compare weight gain upon feeding different feeds. Usually growth trials involve ad libitum feeding of the experimental diets and are compared with a standard (basal) diet of known nutritive quality for a period of time.

Total feed consumption and weight gain are monitored and feed efficiency (weight gain per unit of feed; total weight gain/total feed consumed) is calculated. Although easy and simple to conduct and calculate, results obtained can be affected by several factors. Feed palatability and other physical characteristics and nutrient content can affect voluntary feed intake. Similarly, weight gain includes body tissue mass, including ingesta and water, and can lead to errors in calculation, and thus the results obtained may not be a true reflection of the test diet.

Digestion Trials

Evaluation of the feed is more precise if expressed in terms of the digestibility of each of the nutrients rather than its total content in the feed. Therefore, knowledge and understanding of nutrient digestibility is a necessary step in evaluating feedstuffs for ration formulation. Digestion trials are conducted to determine the proportion of the nutrients in the feed that are digested and absorbed from the gastrointestinal (GI) tract.

Animals are fed the test diets for a period of time for several days and fecal samples are collected and are analyzed for excreted nutrient loss. Total nutrient intake from feed and nutrient disappearance at the end of the digestive tract (fecal loss) is calculated. This conventional method of feeding test diets and fecal collection methods are called fecal digestibility, or total tract digestibility test. However, results obtained through this study are not a true reflection of the digestibility of the test diet. A fraction of the nitrogen, fats, carbohydrates, and inorganic elements appearing in the feces is from endogenous sources. Thus fecal samples may contain undigested feed along with sloughed-off intestinal mucosal cells and digestive enzymes. Fecal nutrients originating from sloughed off intestinal cells and unused digestive enzymes of the animal are called endogenous nutrients. Since not all the nutrients in the feces were derived from the test diet, fecal digestibility is termed apparent digestibility, which represents the difference between the amount ingested and the amount appearing in feces.

Indicator Methods

Although not a true measurement, apparent digestibility is used widely. Monitoring feed intake and fecal collection can be time consuming, laborious, and difficult to conduct in grazing animals. To overcome this, nonabsorbed indicators or markers (acid insoluble ash, chromic oxide) are added to the feed employed, and apparent digestibility is calculated as follows. By using this method, by grabbing a sample of feces from the pen (or the rectum), digestibility can be calculated in animals kept in confinement or on grazing ruminants.

Apparent versus True Digestibility

The true digestibility of a nutrient is the proportion of the dietary intake that is absorbed from the GI tract, excluding endogenous contribution. Assessing endogenous contribution is not that easy.

In monogastric animals, in the hindgut, nutrients that are not digested and absorbed in the small intestine may be fermented by bacteria in the caeca and colon. For example, amino acids produced during protein fermentation in the hindgut are not utilized by the host animal as a source of amino acids. Therefore, ileal digestible amino acids provide a better representation of amino acids that become available to the animal than amino acids digested over the total digestive tract. Therefore, the digestibility of protein and amino acids at the ileal level is a valuable characteristic of the nutritional value of feed ingredients.

Ileal digestibility can be determined with the use of animals fitted with an ileal cannula (special tubes fitted surgically at the end of ileocecal junction) or by collecting ileal digesta at slaughter (done in poultry). Because a quantitative collection of ileal digesta is not possible, inert digestibility markers (e.g., chromic oxide) are also used to estimate nutrient digestibility. Such markers should not interfere with digestive processes, should not be absorbed, and should behave similarly to the nutrient of interest in the GI tract. Although better estimates exist, expertise in surgery and other facilities are needed for these types of measurements.

Due to the high cost of conducting digestion trials, in vitro techniques simulating rumen fermentation are conducted in ruminant animals fitted with rumen fistula. These methods are used for screening feedstuffs or studying rumen function and metabolism.